Relay Beamforming in Cognitive Two-Way Networks with Imperfect Channel State Information

A cooperative bidirectional relaying scheme based on relay beamforming is proposed for an underlay cognitive relay network (CRN) which consists of two secondary transceivers and K cognitive relay nodes and a primary network (PN) with a transmitter and receiver, all equipped with single antenna. The result of incoordination between PN and SN is that the instantaneous channel gains of interference links between two networks are unavailable. So, it is reasonable to assume the availability of second-order statistics (SOS) of the channel coefficients between the relays and PN. Also the perfect channel state information (CSI) is assumed for SN itself. In this paper we find the beamforming coefficients based on requirements of PN or SN. To do so we propose signal-to-interference-plus-noise ratio (SINR) balancing and interference minimization approaches. Our simulation results show that the performance of the network can be degraded considerably by uncertainty about the channel coefficients.